The present invention relates generally to an exposure apparatus, and more particularly to multilayer film structure used for an optical element of an exposure apparatus. The present invention is suitable for an exposure apparatus that exposes an object, such as a single crystal substrate and a glass plate for a liquid crystal display (“LCD”) by using extreme ultraviolet (“EUV”) light with a wavelength of approximately 11 nm.
In a projection exposure apparatus that manufactures a semiconductor device as a semiconductor memory and a logic circuit, not only fine fabrication but also improvement of productivity and safety of operation are demanded. The method of shortening an exposure wavelength is used generally as a means to fabricate the fine pattern and to improve the resolution of the exposure apparatus. Recently, an EUV exposure apparatus using EUV light with a wavelength of 10 to 15 nm has been proposed to efficiently transfer a very fine circuit pattern. Especially, a light with a wavelength of approximately 13.5 nm is generally widely used.
As the light absorption in a material greatly increases in a wavelength range of the EUV light, making impractical a refraction-type optical system for visible light and ultraviolet light, a reflection-type or catoptric optical system is used for the EUV exposure apparatus.
EUV exposure apparatus uses a multilayer mirror that can obtain high reflectivity as a reflection-type optical element. The multilayer mirror, that alternately forms or layers two kinds of materials on a plane substrate, and a number of lamination layer is about 40 layers. The multilayer mirror is used for not only the reflection-type optical element but also a reflection-type mask. In this case, the mask as an original edition uses a reflection-type mask that formed a pattern to be transferred with an absorber on a mirror, and a reflection-type mask that fabricated a pattern to be transferred to a mirror.
The combination of the laminated films is selected according to the wavelength of the EUV light. For instance, if the wavelength is approximately 13.5 nm, the multilayer mirror that combines the molybdenum (Mo) layer and the silicon (Si) layer has the highest reflectivity. Therefore, when the wavelength is approximately 13.5 nm, Mo/Si is generally used for the multilayer (for instance, referring to Japanese Patent Application, Publication No. 1-175736).
However, it is necessary to improve throughput greatly because of the recent demand for the improvement of the productivity. There is a demand for light with a light intensity greater than a light intensity of approximately 13.5 nm wavelength light. Therefore, the use of the wavelength of approximately 11 nm that has a light intensity 2.2 times larger than the light intensity of 13.5 nm is examined (for instance, referring to U.S. Pat. No. 6,228,512). In this case, when the multilayer film that consists of the beryllium (Be) and the molybdenum (Mo) are used for the multilayer mirror, the reflectivity is the highest, and use of the reflection-type optical element and the reflection-type mask is proposed.
However, Be has limitations based on the safety concerns in it's safety (for instance, referring to Chemical safety (hazard) evaluation sheet (Official gazette notification reference No. 1-284, Pollutant Release and Transfer Register), CAS No. 7440-41-7). Therefore, mask containing Be needs to handle a large exhaust amount. In this case, the reflection-type mask that uses Be is especially a problem. A reflection-type optical element that is part of the projection optical system is arranged inside of a vacuum chamber of the exposure apparatus and sealed, so that it is not especially a problem because the exchange frequency is small when the reflection-type optical element is arranged in the vacuum chamber. On the other hand, because the reflection-type mask is exchanged for a different mask in each process, there are many used masks. Therefore, safety is a problem because a frequency that the reflection-type mask is removed to outside of the vacuum chamber increases.